The present invention relates to an optical information storage medium such as an optical disc, an optical tape, an optical card, or the like for recording optical information therein and reproducing optical information therefrom.
There are known optical information storage media comprising a transparent substrate having parallel, concentric, or spiral tracking guide grooves and information tracks disposed between adjacent ones of the guide grooves, the information tracks containing a preformat such as of address information in the form of discrete grooves or pits. The grooved surface of the optical information storage medium is coated with a light-absorbing/reflecting recording film. One such optical information storage medium is illustrated in FIG. 13 of the accompanying drawings.
The optical information storage medium, generally designated by the reference numeral 100, comprises a transparent substrate 100A and an optical recording film or layer 100B. The transparent substrate 100A has tracking guide grooves G defined in one surface thereof. The guide grooves G may be parallel, concentric, or spiral in shape dependent on the form of the optical information storage medium 100. Where the optical information storage medium 100 is disc-shaped, the guide grooves G are concentric or spiral. For the purpose of illustration only, the optical information storage medium 100 shown in FIG. 13 is in the shape of a disc, and the guide grooves G are concentric though they are shown as straight grooves.
Areas L between the guide grooves G are used as information tracks containing address information as a preformat in the form of discrete grooves or pits PL. The information tracks include pit-free flat areas referred to as lands.
The recording film 100B is capable of absorbing and reflecting light and is coated on the surface of the transparent substrate 100A in which the guide grooves G are defined. The recording film 100B may comprise a dye coated film, a dye evaporated film, a metallic film, a metal alloy film, a film of slightly oxidized product of metal, or the like. Optical information is recorded as small holes or phase changes of different reflectivities in the recording film in the lands of the information tracks L.
A laser beam H is applied through the transparent substrate 100A and focused on the recording film 100B for recording or reproducing optical information. No matter whether optical information is to be recorded on the storage medium or to be reproduced from the storage medium, the laser beam H must be properly guided on the information tracks. Control for guiding the laser beam along the information tracks is known as tracking control. Tracking control will briefly be described with reference to reproduction of recorded optical information.
As shown in FIG. 14 of the accompanying drawings, the laser beam H emitted from a laser beam source is reflected downwardly by a deflecting prism 50 and passes through a quarter-wave plate 60 to an objective lens 70. The laser beam H is focused by the objective lens 70 onto the recording film through the transparent substrate of the optical information storage medium 100. A light beam reflected by the recording film travels through the objective lens 70, the quarter-wave plate 60, and the deflecting prism 50 and is applied to two light-detecting surfaces 80A, 80B of a light detector, which then produce photoelectrically converted output signals A, B that are applied to an analog adder 90A and an analog subtracter 90B. The light detector may comprise a PIN photodiode, for example.
The adder 90A produces an output signal indicative of (A+B), and the subtracter 90B produces an output signal indicative of (A-B). The output signal (A+B) from the adder 90A is an RF signal which is representative of the information recorded on the optical information storage medium 100. The output signal (A-B) is a tracking signal (more precisely, a tracking error signal). The tracking control is effected to move the laser beam relatively to the optical information storage medium under servo control so that the tracking signal will be zero.
Since the optical information storage medium has tracking guide grooves on its surface coated with the recording film, the reflected laser beam is subjected to a phase difference dependent on the position where it is reflected, resulting in an interference. The light-detecting surfaces 80A, 80B detect a far-field image of the interference of the reflected laser beam. A variation in the pattern of the far-field image is detected as the tracking signal.
Heretofore, a tracking control failure tends to occur in regions of the optical information storage medium 100 where the preformat is formed.
Generally, the pitch of guide grooves of the optical information storage medium and the spot diameter of the laser beam at the intensity that is 1/e.sup.2 of the maximum intensity are substantially equal, i.e., normally 1.6 micrometers. In this case, correct tracking control requires an accuracy of .+-.0.1 micrometer. The term "correct tracking" means tracking whereby the reduction in the intensity of the RF signal is small, the crosstalk is low, and the danger of the laser beam moving out of the information tracks is small.
The transparent substrate of the optical information storage medium is made of plastics. When the transparent substrate is fabricated of plastics according to the present fabrication technology, however, the transparent substrate is inevitably deformed slightly due for example to warpage or the like. During rotation of the optical information storage medium, it is tilted with respect to the optical axis of the optical pickup by up to about 40 minutes.
With the tilt of 40 minutes, a tracking signal produced from the preformat area by the tracking control system shown in FIG. 14 contains (a) an error ranging from 0.09 to 0.12 micrometer in the case of guide grooves and pits having a rectangular cross section that are difficult to form, or (b) an error ranging from 0.18 to 0.24 micrometer in the case of guide grooves of a V-shaped cross section that are relatively easy to form and pits of a rectangular cross section. The tracking servo control inherently suffers an error of about 0.03 micrometer. Therefore, the error in the preformat area, which is the sum of the error of the tracking signal and the error of the tracking servo control, is in excess of 0.1 micrometer which is an allowable error for the tracking control. As a result, no sufficient tracking accuracy is obtained, and a tracking control failure is liable to happen.
A tracking control failure in the preformat area is also caused in relation to the duty ratio in the preformat area. As shown in FIG. 15 of the accompanying drawings, an optical disc 6 of an optical disc device has a preformat area W in which an address signal, a synchronizing signal, and the like are recorded, the preformat area having pits 10P and lands 11L which are arranged alternately with each other. The duty ratio of the preformat area, i.e., the ratio of the length X of a pit 10P to the sum Y of the length X of the pit 10P and the length of an adjacent land 11 is about 50%. Where the beam spot is focused on the pits 10P, almost all light applied to the pits is diffracted, and the detected light reflected from the pits is of a weak intensity, with the consequence that the information signal detected as the sum signal is of a low level, as shown in FIG. 16. Where the beam spot is focused on the lands 11L, almost all light applied to the lands is reflected since the surface of the lands is of a mirror finish, and the level of the detected information signal is high. The successive high- and low-level signals serve as a preformat signal PB, which is better as the difference between the lower and higher levels is greater.
As illustrated in FIG. 17, when the beam spot is applied to a pit 10P in the preformat area W and focused as indicated as a solid-line spot P, no tracking error signal is produced since the light intensity difference is zero as described above. However, when the light beam is not focused as indicated as an imaginary-line spot Q, a tracking error signal is generated because the light intensity difference is not zero. If a tracking error signal is produced, the objective lens 70 (FIG. 15) is moved in the direction of the arrow C to focus the beam spot on the pit 10P.
In the case where the beam spot is applied to a land 11L in the preformat area W, if the applied beam is perpendicular to the disc surface, then no tracking error signal is produced since there is no difference between the intensities of light detected by the light-detecting surfaces 80A, 80B. If the optical disc 6 is tilted due to warpage, then the reflected light travels in different directions, causing light intensities detected by the light-detecting surfaces 80A, 80B to differ from each other even when the beam spot is not deviated from the track, so that a tracking error signal is produced. When this happens, the beam spot is moved by the servo control system regardless of the fact that the beam spot is in a proper position. Inasmuch as the duty ratio is about 50% in the preformat area, i.e., the length of the land 11L and the length of the pit 10P are substantially equal to each other in the preformat area, the tracking error signal is so much apt to suffer a greater error.
Japanese Laid-Open Patent Publication No. 61-5453 discloses an arrangement in which the error of a tracking error signal is reduced by selecting the duty ratio of the preformat area to be 75% or more, i.e., X/Y .times.100&gt;75 (%). The disclosed arrangement is however limited to a certain system for modulating a preformat signal, and is not suitable for other modulation systems such as FM, MFM, 2-7 modulation, M2 modulation, 8-10 conversion, 4-5 conversion, and the like.
Where the recording film or layer 21 of metal, alloy, or dye capable of absorbing and reflecting light is coated using a solvent, the recording film or layer 21 has a thickness D at the grooves and a thickness d at the lands, as shown in FIG. 15. Since the recording film 21 at the lands is thicker than at the grooves, the lands are more susceptible to deterioration upon exposure to a laser beam.
The recording film deposited in a groove has a shape which is not identical to the shape of the groove defined in the transparent substrate, because the recording film in the groove fails to have sharp corners comprementary to those of the groove, but tends to have dull or blunt corners.